Blazor WebAssembly Azure B2C Error: AADB2C90205 (insufficient permissions)

So it turns out the issue was actually in the code.

I missed a note section in the docs that said I had to manually edit the client app code if API ID URI was untrusted publisher domain similar to https://contoso.onmicrosoft.com/41451fa7-82d9-4673-8fa5-69eff5a761fd, which it was in my case.

Removing the extra https://{TENANT DOMAIN}/ in the Program.cs of the client app fixed things (note the difference between the commented-out line and the line that comes after).

namespace BlazorClient
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            ...

            builder.Services.AddMsalAuthentication(options =>
            {
                builder.Configuration.Bind("AzureAdB2C", options.ProviderOptions.Authentication);
                //options.ProviderOptions.DefaultAccessTokenScopes.Add("https://***.onmicrosoft.com/https://***.onmicrosoft.com/*******/blazor.client");
                options.ProviderOptions.DefaultAccessTokenScopes.Add("https://***.onmicrosoft.com/*******/blazor.client");
            });

            await builder.Build().RunAsync();
        }
    }
}